This invention relates to a veneer cutting machine for tree trunks with variable rake.
As is well known to those skilled in the art, in cutting veneer from trunks a very important parameter for obtaining a good result is the cutting rake of the veneer cutting machine blade, i.e. the angle formed by the plane of the blade in contact with the trunk with the plane tangent to the cylinder constituted by the trunk along the generating line coinciding with the blade edge.
Said angle, hereinafter indicated for simplicity by .alpha., must be established on the basis of the characteristics of the tree species to be veneercut, the heat treatment and the degree of seasoning undergone thereby, and the various other parameters such as trunk diameter which affect the veneer cutting operation.
Dependence on trunk diameter is very important because it is a parameter which varies continuously during veneer cutting. The reason for such dependence is the fact that, to remove from the trunk a continuous strip or even detached sheets of veneer with predetermined, constant thickness, the veneer must be compressed between the point of the blade which performs the cutting and an edge of a special prismatic bar (termeed `torsion bar`) which is positioned near the blade and has the function of stabilizing the penetration of the blade into the wood at the point of separation from the body of the trunk. But the forces generated by compression of the veneer tend to deflect the point of the blade toward the trunk, which must find support in contact therewith, to prevent vibrations which would have a negative effect on the thickness and surface quality of the veneer.
Said support function is obtained by contact on the surface of the trunk of a portion, as long as the trunk and a few millimeters wide, of the side of the blade near the edge thereof.
The need for uniform quality and thickness of the veneer requires that the part of the blade in contact with the trunk remain as much as possible of uniform dimensions and close to those which give minimal vibrations. Contrarywise, if the blade should hold inclination angle .alpha. constant, as the veneer cutting proceeded and consequently the curvature of the trunk increased there would be a decrease in the area of contact between the blade and the trunk.
To avoid this the angle .alpha. must therefore vary with the trunk diameter. Assuming that the veneer cut trunk maintains its cylindrical shape even near the blade contact point, the mathematical relation between radius R of the trunk, angle .alpha. and the width "a" of the contact area will be: EQU .alpha.=-arcosin (a/2R)
But in practice the elasticity of the wood causes distortion thereof in the blade contact area, a distortion which causes an increase of the contact area proportionate to the trunk diameter.
To correct this deviation from the theoretical value given by the above formula, the angle .alpha., always negative therein, must be made positive for a large R and a negative values below a certain value Ro at which .alpha.=0 so as to obtain an effective virtual uniformity of the value of "a" according to R. This requires that the law of change of .alpha. be in practice: EQU .alpha.=.alpha..sub.0 -arcosin (a/2R)
where .alpha..sub.0 =arcosin (a/2R.sub.0).
Thus, mechanisms which in accordance with said law continually and automatically adjust the rake according to the diameter of the trunk being processed have been developed in the known art.
One solution in the known art provides that the blade and pressure bar be mounted on a support moving along appropriately shaped guides so that according to the decrease in the diameter of the trunk the support rotates around the axis passing through the blade edge and causing .alpha. to change in accordance with the formula last written above. In addition, a second group of adjustable guides makes it possible to set the minimum and maximum values of .alpha. for the minimum and maximum values of the radius R so as to be able to adapt the machine for processing woods with different characteristics.
But such a system requires a group of guides with relatively complex operation and is therefore costly and also is not always simple to regulate.